High strength steel sheets are widely used as a blank for making components of an automobile body in order to achieve both a further improvement in the strength, stiffness, and collision safety of the automobile body and an improvement in the fuel economy resulting from the reduced weight of the body. However, the press-formability of steel sheets decreases with increasing strength. As a result, high strength press-formed articles having a desired shape may not be produced.
In recent years, hot press-forming methods (also referred to as hot stamping methods) have been utilized as methods for press-forming components of an automobile body. In hot press-forming methods, a steel sheet (blank) for hot stamping to be press-formed is heated to a temperature equal to or greater than the Ac3 temperature, and immediately after that, is subjected to forming and rapid cooling by a pressing die to be quenched (also referred to as die quenching). In this manner, high strength press-formed articles having a desired shape are produced.
Production of high strength hot press-formed articles in large volumes by a hot press-forming method requires use of a heating furnace for heating steel sheets for hot stamping. Inventions relating to such heating furnaces have been proposed heretofore.
Patent Document 1 discloses a multi-stage heating furnace. The multi-stage heating furnace includes a plurality of accommodation spaces for accommodating a plurality of steel sheets for hot stamping. The plurality of accommodation spaces are aligned in a vertical direction so as to be horizontal to each other. Means for transferring the steel sheets for hot stamping during heating are provided in the plurality of accommodation spaces.
Patent Document 2 discloses a multi-stage heating furnace that includes a box-shaped body and a heat source. Heating chambers are formed within the body. The heat source heats the insides of the chambers to about 900° C. This multi-stage heating furnace is capable of heating a plurality of steel sheets for hot stamping simultaneously and discharging the heated steel sheets for hot stamping separately.
Patent Document 3 discloses a multi-stage heating furnace that includes a body. Heating chambers to be heated by heat sources are provided within the body. Multiple-staged openings arranged in a vertical direction are provided in the front wall of the body. An opening and closing door is provided for each opening at each stage.
Furthermore, Patent Document 4 discloses a heat treatment method. The heat treatment method includes a first step and a second step. In the first step, a steel sheet for hot stamping is heated to an alloying temperature. In the second step, a first region of the steel sheet for hot stamping is held at a temperature equal to or greater than the A3 transformation temperature utilizing thermal energy imparted in the first step while depriving a second region of the steel sheet for hot stamping of thermal energy. As a result, the second region of the steel sheet for hot stamping cools to a temperature equal to or less than the A1 transformation temperature. This heat treatment method can effectively utilize thermal energy imparted in the alloying process and shorten the time for heat treatment.
The heating furnaces disclosed by Patent Documents 1 to 4 use a gas burner, an electric coil heater, a radiant tube, an electromagnetic heater, or another type of heater as the heat source for steel sheets for hot stamping.
These heating furnaces need to meet the following requirements: rapid and uniform heating of the steel sheet for hot stamping over all regions to a high temperature range of equal to or greater than the Ac3 temperature (e.g., from 850 to 950° C.); an improvement in the ability for mass production; and minimization of the area for installation. In recent years, heating furnaces utilizing a far-infrared radiation heater as its heat source have been increasingly used. Heating furnaces of this type have the characteristics a to c listed below:
(a) capable of uniformly heating a steel sheet for hot stamping;
(b) capable of being compact by virtue of the vertically extending multi-stage configuration; and
(c) having a thin planar shape and being capable of heating a steel sheet for hot stamping at both sides thereof.
Patent Document 5 discloses a multi-stage heating furnace using a flexible far-infrared radiation heater as its heat source. The flexible far-infrared radiation heater is constructed of numerous insulators arranged in rows and knitted together to form a flexible panel. The numerous insulators have slits for receiving a resistive heating conductor. A heating conductor that emits far-infrared radiation is inserted and provided in the slits.